Such fibers, generally consisting of silica glass, are conventionally provided with resin layers designed to protect their surfaces from dust and other abrasive influences that could impair their light-transmitting quality. Thanks to its lower coefficient of elasticity, such a resin layer also serves to absorb breaking stresses which might otherwise rupture the more brittle fiber core.
By the usual treatment method, the fiber is passed in a downward direction through a mass of liquid resin contained in a reservoir which terminates in a converging outlet port. After the fiber emerges from that port, its resin coating is allowed to set by drying or polymerization.
As is well known in the art, the outlet port should have a certain minimum length in order to insure a laminar flow of the resin, thereby providing a uniform and continuous coating. The thickness of that coating depends, of course, on the outlet diameter which generally ranges between about 150 and 300.mu.. Resins with a high coefficient of elasticity should be applied in a thin layer of up to 5.mu. whereas those of lower coefficients may have thicknesses ranging between 20 and 40.mu.. The convergence of the outlet port is designed to counteract radial force components, resulting from the viscosity and the flow velocity of the resin, which tend to interfere with the concentricity of the coating; the usual vertex angle ranges between 2.degree. and 8.degree..
A nozzle forming both the reservoir and the outlet duct can be made integral or can be split into two symmetrical halves clamped together. In the first instance, the fiber to be coated must be introduced into the nozzle bore before being placed under traction so that its diameter may not have reached its final size; this could cause a blocking of the outlet and a rupture of the fiber. With a two-part nozzle, on the other hand, its halves can be closed around the fiber after the drawing operation has been started.
In either case, the relatively narrow outlet port of the nozzle experiences considerable wear during use, especially in the initial phase when the reservoir does not yet contain any resin; even thereafter, such wear may result from fiber vibrations which are only incompletely damped by the surrounding resin flow. In order to maintain the coating diameter within prescribed tolerance limits, the nozzle must therefore be frequently replaced unless it consists of a highly abrasion-resistant material such as ruby, corundum or tungsten carbide, for example. These materials, however, are very difficult to machine with the precision necessary to produce a very smooth guide surface for the exiting fiber, especially when the nozzle body is split into two parts.